Method and system for dynamic gateway selection in an IP telephony network

ABSTRACT

A method and system for dynamically selecting a destination gateway to complete a call over a path supported at least in part by an IP telephony network and a public switched telephone network. The method and system further provide for dynamically detecting available gateways, dynamically removing failed and/or unavailable gateways, and automatically recovering failed and/or unavailable gateways after a predetermined period of time. A method is also provided for detecting available destination gateways using a ping method, where a message is transmitted to a plurality of destination gateways on a one-by-one basis to ascertain the availability status of each destination gateway.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the field of IP telephony,and more particularly to a method and system for selecting gateway(s)for routing calls through a packet-based telecommunications networkinterconnected with a public telecommunications network.

2. Acronyms

The written description herein uses a number of acronyms to refer tovarious services, messages and system components. Although generallyknown, use of several of these acronyms is not strictly standardized inthe art. For purposes of this discussion, several acronyms will bedefined as follows:

Dynamic Gateway Selection (DGS)—a process employed by a network serverand a Redirect Server (RS) to allow calls to be routed to an egressgateway.

Egress (Destination) and Ingress (Origination) Gateways—gatewaysconnecting an IP network to a PSTN, PBX or other network.

Network Management System (NMS)—performs a variety of functions,including receiving and storing status changes to destination or egressgateways.

3. Description of the Prior Art

Internet telephony provides real-time delivery of voice, and othermultimedia data, between two or more parties across a network employingInternet protocols (IP). Internet telephony is session-based rather thanconnection-based. That is, Internet telephony uses virtual connectionsor circuits to pass data between two nodes. These connections betweenthe nodes are termed “virtual circuits” to distinguish them fromdedicated circuits of conventional networks.

While IP telephony offers benefits to both users and carriers in termsof costs and variety of media types that can be routed therethrough,there are a substantial amount of traditional telephones being servicedby the Public Switched Telephone Network (PSTN). The PSTN provides userswith dedicated, end-to-end circuit connections for the duration of eachcall. Circuits are reserved between an originating switch and aterminating switch based on a called party number.

However, because of the popularity of the Internet, many publictelecommunications networks now carry significantly more IP data trafficthan voice data traffic. Public telecommunications networks, optimizedfor voice traffic, are ill-equipped to handle increasing data trafficvolumes. The growth in IP data traffic coupled with customer demands forintegrated voice and data services at lower costs has led to theadoption of IP as the preferred protocols for carrying both voice anddata traffic between originating and terminating switches of the publictelecommunications network.

Accordingly, in order for IP based telephony services to become broadlyaccepted by users of traditional telephones, it is necessary tointerface the IP telephony network with the existing PSTN and withprivate PBX phone networks. To permit this mode of operation, apacket-based network, such as the Internet, must interface directly withpublic telephone networks and operate as a bridge between originatingand destination switches of such networks.

Media streams which originate from a public network must be capable ofbeing transported through the packet-switched IP network and terminateat the same or different public network. This type of interfacing isperformed by gateways which interface the signaling and bearer pathsbetween the two networks. Therefore, Internet gateways perform code andprotocol conversion between two otherwise incompatible networks toprovide links necessary to send packets between the two differentnetworks and thus make network-to-network connections possible. Toassure overall system reliability it is crucial that the gateways arereliable and their availability, especially destination or egressgateways, be monitored for quickly and efficiently selecting anavailable gateway.

SUMMARY OF THE INVENTION

In accordance with the principles of the invention, a method and systemare provided by which an egress (i.e., destination) gateway isdynamically selected to establish a communication session over a pathsupported at least in part by an IP telephony network and a PSTN, PBX orother network. A redirect server (RS) in concert with a networkmanagement system (NMS) employs a gateway selection methodology whichincludes recording egress gateways which are not available due toseveral reasons, such as having timed out, or having a malfunction,i.e., failure status.

In one embodiment of the present invention, a method is provided fordynamically selecting an egress gateway to allow a call to be completedin a communication session over a path supported at least in part by anIP telephony network and a PSTN, PBX or other network. The IP telephonynetwork includes a plurality of ingress and egress gateways, at leastone Session Initiation Protocol (SIP) proxy server (SPS) and at leastone redirect server (RS). A dynamic gateway selection (DGS) feature isalways active and is typically invoked whenever a source user agent(SUA) initiates a call attempt by sending a session participationrequest to the SPS.

The preferred method generally includes the steps of: receiving a callsetup request at the SPS from the SUA. The SPS forwards the request tothe RS to obtain information of destination gateways. The RS responds tothe SIP session participation request with either a redirection responseor a request failure response. The RS redirection response includes arouting list. The routing list is a list of egress (i.e., destination)gateways that are determined to be in-service. Upon receipt of aredirection response from the RS, the SPS proxies the sessionparticipation request to a first destination gateway in the routinglist. Otherwise, the RS returns a failure response which is sent to theSUA. The failure response is an indication that there are no destinationgateways having an in-service status.

When the SPS proxies the request to a destination gateway, the SPS waitsfor a final response from the selected destination gateway. The SPS willeither receive a final response or time-out. When the SPS receives afinal response from the destination gateway, the SPS proxies the finalresponse to the SUA, awaits an acknowledgment and proxies theacknowledgment. Otherwise, when the SPS times-out waiting for a finalresponse from the destination gateway, the SPS re-sends the request apredetermined number of times. If the SPS times-out for a final time,the SPS sends the session participation request to the next destinationgateway in the routing list provided by the RS.

The process of sending a request a predetermined number of times isrepeated for the next destination gateway in the routing list until oneof the following occurs: (1) a success response is returned; (2) the SPStimes-out waiting for a final response, as described above with respectto the first destination gateway in the routing list; (3) the SPSreceives an unsuccessful final, non-server failure response from thecurrently selected destination gateway; or (4) the destination gatewayreturns a server failure response, in which case the SPS informs the RSof the destination gateway failure.

In case of the second to fourth situations, the SPS sends the sessionparticipation request to other destination gateways in the routing listprovided by the RS. For each destination gateway that returns a gatewayfailure response to the SPS, the destination gateway is recorded as anout-of-service destination gateway in the RS and is dynamically removedfrom the routing list. Therefore, subsequent requests are not sent todestination gateways which are recorded as out-of-service destinationgateways in the RS until after a predetermined amount of time. After thepredetermined amount of time has elapsed, the RS automatically recoversfailed or out-of-service pathways and issues a report to a networkmanagement system (NMS) indicating that the destination gateway is backin service. Table structures, stored within the RS, are updated on areal-time basis when it is determined that a gateway is out-of-serviceor back in-service. When the session participation request has been sentto all destination gateways and no successful response is received, theSPS returns a final response to the originating agent or calling partyindicating that a call cannot be made.

In an alternative method, availability of destination gateways isdetermined using a ping method. In this method, gateway availability isdetermined by sending at least one packet to each destination gateway toascertain whether the destination gateway is available, or in-service.If the destination gateway is in-service, it transmits an ACK message.If an ACK message is not received after a predetermined period of time,the destination gateway (DGW) is determined to be unavailable. The pingmethod preferably queries each destination gateway one-by-one andupdates gateway information tables by recording the status of eachdestination gateway.

The present invention thereby provides several functions, including: (1)dynamic detection of failed and unavailable gateways; (2) dynamicremoval of failed and unavailable gateway(s) from a routing list,gateway information table, etc., after a predetermined period of time;and (3) automatic recovery of failed and unavailable gateways byupdating gateway status tables after a predetermined period of time.

BRIEF DESCRIPTION OF THE DRAWINGS

Various preferred embodiments are described herein with reference to theaccompanying drawings, in which:

FIG. 1 is a block diagram of a preferred embodiment of the presentinvention;

FIG. 2 is a flowchart illustrating signaling and call setup proceduresaccording to the embodiment of FIG. 1;

FIG. 3 is a call flow diagram illustrating the signaling and call setupprocedures according to the embodiment of FIG. 1; and

FIG. 4 is a flowchart illustrating an alternate embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will now be describedwith reference to the figures where like reference numbers indicateidentical or similar elements. It will be apparent to persons skilled inthe relevant art that the present invention can operate on manydifferent types of networks without departing from the scope of thepresent invention. In the preferred embodiments described herein, the IPtelephony network is preferably the Internet. Other examples of networkswhich could be used include leased lines, frame delay, and asynchronoustransfer mode (ATM) networks.

The present invention provides a method and system for selecting anegress or destination gateway to establish a communication session overa path supported at least in part by an IP telephony network, such as aWAN, and a PSTN, PBX or other network. The method further determines thestatus of a destination gateway, particularly, as being eitherin-service or out-of-service, and automatically brings a destinationgateway back into service from an out-of-service state after apredetermined amount of time.

Referring now to the drawings, and first to FIG. 1, a system accordingto a preferred embodiment of the present invention is designatedgenerally by reference numeral 10. System 10 is a telephony networksystem and includes a first public switched telephone network (PSTN) 114a which interfaces to an IP telephony network or Internet 112. TheInternet 112 is further interfaced to a second PSTN 114 b. The firstPSTN 114 a includes a source user agent (SUA) 102, i.e., originatingagent, which originates a session participation request. The second PSTN114 b includes a called party destination user agent (DUA 103). TheInternet 112 includes a redirect server (RS) 104, an SIP proxy server(SPS) 106, a Network Management System (NMS) 108, and destinationgateways (DGWs) 110 a, 110 b. While only two DGWs are shown, one ofordinary skill in the art will recognize that additional DGWs may beprovided. The RS 104 supports a gateway management function which tracksthe status of the DGWs. The NMS 108 receives and stores all statuschanges regarding DGWs 110 a, 110 b. Status changes are reported to theNMS 108 by the RS 104 via the SPS 106. SPS 106 acts as both a server andclient for the purpose of making requests on behalf of other clients.SPS 106 provides proxy server and gateway resource management functions.SPS 106 may be a SIP proxy server or an H.323 gatekeeper.

The method of the present invention (i.e., dynamic gateway selection(DGS) and removal) is performed by the SPS 106 and RS 104 in contextwith the NMS 108 in order to allow calls to be routed to one of the DGWs110 a, 110 b. The dynamic gateway selection and removal feature isparticularly invoked upon receipt by the RS 104 of a sessionparticipation request. The SUA 102 initiates a call attempt to transmitthe session participation request to the SPS 106. Accordingly, anattempt is made to establish a communication session between the SUA 102located in the first PSTN 114 a and the called party destination device(DUA) 103 located in the second PSTN 114 b. PSTN 114 a and 114 b arebridged via the Internet 112.

FIG. 2 is a flowchart illustrating the call routing logic forestablishing a communication session in accordance with the methodologyof the present invention. At step 702, a user initiates a call attemptby sending a session participation request (i.e., an INVITE request) tothe SPS 106. When the INVITE request is an initial request, the SPS 106forwards the initial INVITE request to the RS 104 for routinginstructions at step 704. At step 705, it is determined whether there isat least one DGW that can satisfy the request. If so, at step 706, theRS 104 responds to the SPS 106 query with a routing list, i.e., a listof candidate DGWs that can handle the call. The RS 104 supplies therouting list from a gateway information table stored therein.

In the event the RS 104 determines that there are no DGWs that cansatisfy the INVITE request, at step 705, a request failure response isreturned to the SUA 102, at step 707. Upon receiving the routing list,the SPS 106 proxies the INVITE request to one of the DGWs 110 a, 110 b.At step 708, the SPS 106 selects the first DGW 110 a in the routing listto determine its serviceability and/or availability status. Steps 710and 712 determine whether the currently selected DGW 110 a from therouting list is in a failure state or has timed out. Specifically, atstep 710, a determination is made concerning whether the DGW 110 areturns a failure response (i.e., out-of-service response). If there isa failure response at step 710, the SPS 106 reports the gateway failureto the RS 104 at step 714. The RS 104 marks the selected DGW 110 a as anout-of-service destination gateway in a gateway information table storedin the RS 104, at step 716.

In addition, the SPS 106 sends a message, (i.e., Simple NetworkManagement Protocol (SNMP) trap), to the NMS 108 indicating adestination gateway failure. The SPS 106 then selects the next DGW 110 bin the routing list, at step 718. Control then returns to step 710 todetermine the availability of the next selected DGW 110 b; that is,whether the next selected gateway 110 b is in a failure state or hastimed out. If the next selected DGW 110 b returns either a failureresponse at step 710, or times-out at step 712, then steps 714-718 arerepeated. In short, the process of selecting a gateway from the routinglist and determining whether it is in a failure state or has timed outis repeated until a DGW is found from the routing list which accepts acall with a success response at step 720. When a success response isreceived, the SPS 106 forwards the response to the calling user SUA 102at step 722. The media stream for the call is then set up at step 724 toestablish a communication link between the SUA 102 and DUA 103.

FIG. 3 is a call routing flow diagram illustrating in greater detail thecall routing logic procedure described above with reference to FIG. 2.Table 1 below lists the INVITE required parameter fields in thepreferred embodiment when a SUA 102 initiates a call attempt by sendinga session participation request (i.e., an INVITE request) to the SPS 106(See FIG. 1, item 1 and FIG. 3, step “a”).

TABLE 1 Parameter Usage Request-Line Contains the method (e.g., INVITE),Request Uniform Resource Identifier (i.e., Request-URI) of the SPS andprotocol version To Contains the address of the recipient of the requestFrom Contains the address of the initiator of the request Call-IDUniquely identifies the invitation Cseq Contains the request method anda decimal sequence number chosen by the requesting client, unique withina single value of the Call-ID Via Indicates the path taken by therequest so far

The SIP INVITE is addressed to the called party DUA 103 at a proxyaddress at the SPS 106. The SIP INVITE specifies the real IP address ofthe DUA 103. Upon receipt of the SIP INVITE, the SPS 106 sends a 100trying message to the ingress or origination gateway (FIG. 3, step “b”).Table 2 lists the mandatory fields associated with the 100 tryingresponse message in the preferred embodiment.

TABLE 2 Parameter Usage Status-Line Status Code = 100, Reason phrase andprotocol version To Content copied from the original request messageFrom Content copied from the original request message Call-ID Contentcopies from the original request message Cseq Content copies from theoriginal request message Via Indicates the path taken by the request sofar. Add the received-tag parameter if the previous ho address isincorrect in the via header field

The SPS 106 counts the number of INVITE requests received. When areceived request message does not contain a route header field, it isdetermined to be an initial INVITE request message. In such a case, theSPS 106 performs the following steps: (1) if a Topmost Via Header (TVH)source address is incorrect, adds a “Received” parameter (or replacesthe existing one if there is one) with the source package to the Viaheader field inserted by the previous hop; (2) generates an internalCall-ID; or (3) forwards the requested message to the RS 104 (See FIG.1, item 2 and FIG. 3, step “c”). Table 3 lists the required fields inthe RS INVITE request message.

TABLE 3 Parameter Usage Status-Line Content copied from the originalrequest message To Content copied from the original request message FromContent copied from the original request message Call-ID Internallygenerated Call-I Cseq Content copied from the original request messageVia Add the received tag

In response to receiving the RS INVITE request message from the SPS 106,the RS 104 performs the following: (1) counts the number of INVITEmessages received; (2) determines that the user portion of RequestUniform Resource Identifier (i.e., (Request-URI) is less than or equalto 15 digits and does not contain a leading 0 or 1; and (3) queries aNetwork Control System/Data Access Point (i.e., NCS/DAP) to obtainrouting information (FIG. 3, step “d”). Routing information, including arouting list of prospective DGWs, is then sent from the NCS/DAP to theRS 104 (FIG. 3, step “e”). The routing information is used to update thegateway information table stored in RS 104. The gateway informationtable is subsequently used to create an updated routing list.

For example, when a gateway address is marked as out-of-service in thegateway information table stored in RS 104 and its associated time valueis zero the gateway address is not added to the routing list. When thegateway address is marked as out-of-service in the gateway informationtable and its associated time value is greater than the current absoluteRS time, the gateway address is not added to the routing list. When thegateway address is marked as out-of-service in the gateway informationtable and its associated time value is less than or equal to the currentabsolute RS time, the gateway address is added to the routing list andthe gateway address is marked as in-service in the gateway informationtable. The RS 104 also sends a message, i.e., the Simple NetworkManagement Protocol (SNMP) trap, to the NMS 108 indicating that the DGWis in-service. If there is only one gateway in the routing list, the RS104 will send a 302 response message back to the SPS 106 (See FIG. 1,item 3 and FIG. 3, step “f”). The RS 104 increments a counter whichcounts the number of 3xx messages sent. Table 4 lists the requiredfields in the 3xx (302 in the present case) response message and anexample of the contact address list.

TABLE 4 Parameter Usage Status-Line Status Code = 302, Reason phrase andprotocol version To Content copied from the Original INVITE request FromContent copied from the Original INVITE request Call-ID Content copiedfrom the Original INVITE request Cseq Content copied from the OriginalINVITE request Via Content copied from the Original INVITE requestContact Multiple gateway addresses

Table 5 lists the required fields in the SNMP trap message to the NMS.

TABLE 5 Parameter Usage Protocol Data Indicates that this is a Trap PDUnit (PDU) Type Enterprise Identifies the network-management subsystemthat generated the trap. Its value is taken from sysObjectID in thesystem Group Agent-addr The IP address of the object generating the trapGeneric-trap 6 = enterpriseSpecific. This value signifies that thesending protocol entity recognizes that some enterprise-specific eventhas occurred. The specific-trap field indicates the type of trapSpecific-trap A code that indicate more specifically the nature of thetrap Time-stamp The time between the last (re)initialization of thenetwork entity that issued the trap and the generation of the trapVariable The address of the gateway that returned the 5xx bindingsresponse and status (in-service)

In the case where there is more than one gateway in the routing list,the RS 104 sends a 300 response message, instead of a 302 responsemessage for a single gateway, back to the SPS 106. For a 300 response,the RS 104 also counts the number of 3xx responses sent. Table 6 liststhe required fields in the 300 response message and an example of thecontact address list.

TABLE 6 Parameter Usage Status-Line Status Code = 300, Reason phrase andprotocol version To Same as the original INVITE From Content copied fromthe Original INVITE request Call-ID Content copied from the OriginalINVITE request Cseq Content copied from the Original INVITE request ViaContent copied from the Original INVITE request Contact Multiplereachable addresses

The SPS 106 counts the number of routing responses received from the RS104. The SPS 106 sends an INVITE to the first DGW 110 a, and inserts the“user=phone” header in each contact list address (See FIG. 1, item 4 andFIG. 3, step “g”). Table 7 lists the required fields of the INVITErequest sent from the SPS 106 to the DGW 110 a.

TABLE 7 Parameter Usage Request-Line Contains the method (e.g., INVITE),Request-URI using the gateway from the top of the unused contact listand protocol version To Content copied from the original request messageFrom Content copied from the original request message Call-ID Contentcopied from the original request message Cseq Content copied from theoriginal request message Via Add the NS URL to the top Record RouteRequest-URI of the NS

The SPS 106 may receive a 100 trying response (see FIG. 3, step “h”) ora 180 ringing response from the DGW 110 a (See FIG. 3, step “i”). Table8 lists the required fields of the 180 ringing response.

TABLE 8 Parameter Usage Status-Line Status Code = 180, Reason phrase andprotocol version To Same as the original INVITE request plus tag FromSame as the INVITE request sent to the Egress Gateway Call-ID Same asthe INVITE request sent to the Egress Gateway Cseq Same as the INVITErequest sent to the Egress Gateway Via Same as the INVITE request sentto the Egress Gateway

After receiving the 180 ringing response from the DGW 110 a, the SPS 106removes itself from the top of the Via field, re-starts the invite UserAgent (UA) timer if it exists, and forwards the 180 ringing response tothe ingress gateway (See FIG. 3, step “j”). The response message is sentto the address indicated in the Via header field. Table 9 lists therequired message elements.

TABLE 9 Parameter Usage Status Line Content copied from the 180 responsereceived from the gateway To Content copied from the 180 responsereceived from the gateway From Content copied from the 180 responsereceived from the gateway Call-ID Content copied from the 180 responsereceived from the gateway Cseq Content copied from the 180 responsereceived from the gateway Via Content copied from the 180 responsereceived with the removal of the NS URL

The SPS receives an INVITE response (i.e., 200 OK response) from the DGW110 a (See FIG. 3, step “k”). Table 10 lists the required fields in theINVITE message.

TABLE 10 Parameter Usage Status Line Status Code = 200, Reason phraseand protocol version To Same as the original INVITE request plus tagFrom Same as the INVITE request sent to the Egress Gateway Call-ID Sameas the INVITE request sent to the Egress Gateway Cseq Same as the INVITErequest sent to the Egress Gateway Record Route Request-URI of the NSContact The reachable address of the Egress Gateway

After receiving the 200 OK response from the DGW 110 a, the SPS 106performs the following steps: (1) cancels the invite UA timer, if itexists; (2) removes the SPS URL from the Topmost Via Header (TMVH)field; (3) adds the next hop's Request-URI at the top of therecord-route header field when either of the following conditions aremet: a) there is no contact header field in the 200 OK response, or b)the SPS URL is on the top entry of the record-route header field; (4)counts the number of 200 INVITE responses sent by the SPS 106; and (5)the SPS 106 starts the ACK timer. The SPS 106 forwards the INVITEresponse (i.e., 200 OK response) to the ingress gateway (See FIG. 3,step “I”). Table 11 lists the required headers in the INVITE response.

TABLE 11 Parameter Usage Status-Line Status Code = 200, Reason phraseand protocol version To Same as the original INVITE request plus tagFrom Same as the INVITE request sent to the Egress Gateway Call-ID Sameas the INVITE request sent to the Egress Gateway Cseq Same as the INVITErequest sent to the Egress Gateway Via Content from the INVITE requestsent to the Egress Gateway Record Route Request-URI of the NS ContactThe reachable address of the Egress Gateway

The SPS 106 receives an ACK response from the ingress gateway and stopsthe ACK timer. The SPS 106 counts the number of ACK response messagesreceived by the SPS 106. Table 12 lists the required headers of the ACKresponse (See FIG. 3, step “m”).

TABLE 12 Parameter Usage Request-Line Contains method (e.g., ACK),Request-URI of the NS and protocol version To Same as the originalINVITE plus the tag From Same as the original INVITE Call-ID Same as theoriginal INVITE Cseq Same sequence number as the original INVITE Via UAoriginated

The received ACK response may contain a route header field. The SPS 106either proxies the ACK response using the address in the route header oruses the address in the “To” header to determine whether to proxy theACK response or consume the ACK response. The ACK response will beconsumed when the “To” header address is equal to the SPS address, andno route header exists in the ACK response. When the SPS 106 determinesthat the ACK response should be proxied, the SPS 106 performs thefollowing: (1) the SPS 106 adds the ACK's address to the top of the Viafield; (2) the SPS 106 removes the top address from the route headerfield; (3) the Request-URI is set to the address located at the top ofthe route header field; and (4) the ACK message is forwarded to the DGW110 a based on the top address in the route header field if it exists orbased on the call context's DGW information (See FIG. 3, step “n”).Accordingly, the DGW 110 a is selected as the available gateway forcompleting the call. If the DGW 110 a is determined to be unavailable,the same method outlined above is used to determine if DGW 110 b isavailable. If neither DGW is available, a message is sent to the SUA 102that the call cannot be completed. Table 13 lists the parameters of theACK request message sent to the DGW 110 a.

TABLE 13 Parameter Usage Request-Line Contains method (e.g., ACK),Request-URI is copied from the top list of Route header field andprotocol version To Content copied from the ACK received from theIngress Gateway From Content copied from the ACK received from theIngress Gateway Call-ID Content copied from the ACK received from theIngress Gateway Cseq Content copied from the ACK received from theIngress Gateway Via UA originated with the NS URL added to the top ofVia field

In another preferred method, a DGW is selected using a ping method. Inthis embodiment, gateway availability is determined by sending at leastone packet to each destination gateway to ascertain whether thedestination gateway is available, or in-service. Accordingly, if thedestination gateway is in-service, it transmits an ACK message. If anACK message is not received after a predetermined period of time, theDGW is determined to be unavailable. The ping method preferably querieseach destination gateway one-by-one and updates a gateway informationtable by recording the status of each gateway. For example, if the ACKmessage is received, the DGW is then checked to determine if it isavailable. If it is available, its address is stored in a gatewayinformation table, and the process repeats for the next DGW.

FIG. 4 is a flowchart illustrating the methodology of an alternateembodiment of the present invention, i.e., the ping method. A counter isinitialized at step 800 to indicate the currently selected destinationgateway from the routing list (i.e., i=1 to the number of gateways inthe routing list). In step 802, a message is transmitted from a server(e.g. proxy server, redirect server) to the ith destination gateway forthe purpose of obtaining a response. In step 804 the server awaits aresponse from the ith destination gateway. Step 806 is a determinationstep to determine whether a response was received from the ithdestination gateway. If a response is not received within apredetermined amount of time, the process continues at step 808 where itis determined whether there are additional destination gateways to checkfrom the routing list. If not, the process terminates at step 810.Otherwise, the counter is incremented to select a succeeding destinationgateway from the routing list at step 812. Steps 802-806 are thenrepeated to check the response and/or availability of the succeeding(i.e. ith+1) destination gateway from the routing list. If it isdetermined at step 806 that a response is received within thepredetermined time, the process continues to step 814 where it is thenfurther determined whether the responding destination gateway isavailable or not. If the destination gateway is not available, theprocess returns to step 808 to determine whether there are additionaldestination gateways to check from the routing list. If so, steps802-806 are repeated for the succeeding destination gateway. Otherwise,if it is determined at step 814 that the responding destination gatewayis available, the process continues at step 816 where the address of theavailable destination gateway is stored in a gateway information table.The process returns to step 808 to determine if there are additionaldestination gateways to be checked in the routing list.

What has been described herein is merely illustrative of the applicationof the principles of the present invention. For example, the functionsdescribed above for operating the present invention are for illustrationpurposes only. Other arrangements and methods may be implemented bythose skilled in the art without departing from the scope and spirit ofthe invention.

1. A method for routing calls to a destination gateway to establish acommunication session call in a telecommunications network between asource user agent and a destination user agent over a path supported atleast in part by a telephone network and an IP network, the IP networkincluding a plurality of ingress and destination gateways, at least oneproxy server, and at least one redirect server (RS), the methodcomprising: receiving a call setup request at the at least one proxyserver from the source user agent, where the source user agent isincluded in a public switched telephone network and the call set uprequest identifies the destination user agent; forwarding the receivedcall setup request to the redirect server; determining, by the redirectserver, a status of a group of destination gateways in response toreceiving the forwarded call setup request, where the status of aparticular destination gateway is determined as one of in-service orout-of-service; adding the particular destination gateway to a routinelist and recording the particular destination gateway as in-service, ifthe status of the particular destination gateway is determined to beout-of-service and if a time value associated with the determined statusis less than or equal to a current time value associated with theredirect server; not adding the particular destination gateway to therouting list, if the status of the particular destination gateway isdetermined to be out-of-service and if the time value associated withthe determined status is greater than the current time value associatedwith the redirect server; receiving the routing list or a requestfailure response from the redirect server; proxying the call setuprequest by the at least one proxy server to a destination gatewayselected from the routing list upon receiving the routing list from theredirect server, where the selected destination gateway is tocommunicate with a public switched telephone network that includes thedestination user agent; upon proxying the call setup request to theselected destination gateway, waiting for a response from the selecteddestination gateway; upon receiving the response from the selecteddestination gateway within a predetermined time, establishing acommunication, session using the selected destination gateway; and ifthe response is not received within the predetermined time, sending thecall setup request to a succeeding destination gateway selected from therouting list and reporting failure of the selected destination gatewayto the redirect server, where the succeeding destination gateway is tocommunicate with a public switched telephone network that includes thedestination user agent.
 2. The method as claimed in claim 1, furthercomprising repeating the proxying the call set up request, the waitingfor a response from to selected gateway, the establishing acommunication session upon receiving the response, and the sending thecall setup request to a succeeding destination gateway if the responseis not received within the predetermined time, until a destinationgateway is determined to be available for establishing the communicationsession or until all destination gateways from the routing informationhave been determined to be unavailable.
 3. The method as claimed inclaim 1, further comprising recording a destination gateway status asout-of-service if the response from the destination gateway is notreceived within the predetermined time.
 4. The method as claimed inclaim 3, where the recording further comprises recording the destinationgateway status as out-of-service in a gateway information table storedwithin the redirect server.
 5. The method as claimed in claim 1, wherethe receiving a call setup request at the at least one proxy server fromthe source user agent includes addressing the call setup request to aproxy address of the at least one proxy server.
 6. The method as claimedin claim 1, where the receiving a call setup request at the at least oneproxy server from the source user agent includes counting a number ofreceived requests subsequent to the call setup request at the at leastone proxy server.
 7. The method as claimed in claim 1, where the atleast one proxy server comprises a Session Initiation Protocol (SIP)proxy server.
 8. The method as claimed in claim 1, where the at leastone proxy server comprises an H.323 gatekeeper.
 9. The method as claimedin claim 1, further comprising: sending a message from the at least oneproxy server to a network manager to record the status of a destinationgateway.
 10. The method as claimed in claim 1, further comprising:forwarding a request failure response to the source user agent uponreceiving the request failure response from the at least one proxyserver; and terminating the communication session.
 11. The method asclaimed in claim 1, further comprising: resending the call setup requestto the selected destination gateway a predetermined number of times whenthe response is not received within the predetermined time.
 12. Themethod according to claim 1, where the routing list includes at leastone destination gateway that can handle the call according to statusinformation tracked by the redirect server.
 13. The method according toclaim 1, where the call setup request identifies the destination useragent by specifying the address of the destination user agent.
 14. Themethod according to claim 13, where the address of the destination useragent includes a real IP address of the destination user agent.
 15. Themethod according to claim 1, where the redirect server tracks status ofat least one destination gateway.
 16. A system for establishing a callbetween a calling party and a called party, the system comprising: afirst telephony system including at least one source user agent (SUA); asecond telephony system including at least one destination user agent(DUA); an internet protocol (IP) network connected between the first andsecond telephony systems; a plurality of ingress gateways to interfacethe IP network to the first telephony system; a plurality of egressgateways to interface the IP network to the second telephony system; anIP telephony proxy server; and an IP redirect server; where the IPtelephony proxy server is to: receive call setup request from the sourceuser agent, where the call setup request identifies the destination useragent; forward the call setup request to the IP redirect server; receiverouting information from the IP redirect server; and select one of theplurality of egress gateways to complete the call based on the receivedrouting information; and where the IP redirect server is to: determine astatus of a group of egress gateways in response to receiving theforwarded call setup request, the status of a particular egress gatewayis determined as one of in-service or out-of-service, add the particularegress gateway to routing information and record the particular egressgateway as in-service, if the status of the particular egress gateway isdetermined to be out-of-service and if a time value associated with thedetermined status is less than or equal to a current time valueassociated with the redirect server, not add the particular egressgateway to the routing information, if the status of the particularegress gateway is determined to be out-of-service and if the time valueassociated with the determined status is greater than the current timevalue associated with the redirect server, and provide the routinginformation to the IP telephony proxy server; and a network managementsystem to receive and store status changes of egress gateways, thenetwork management system being in communication with the IP telephonyproxy server.
 17. The system as claimed in claim 16, where the IPtelephony proxy server includes a Session Initiation Protocol (SIP)proxy server.
 18. The system as claimed in claim 16, where the IPtelephony proxy server includes an H.323 gatekeeper.
 19. The methodaccording to claim 16, where the call setup request identifies thedestination user agent by specifying the address of the destination useragent.
 20. The method according to claim 19, where the address of thedestination user agent includes a real IP address of the destinationuser agent.